Manufacture and production of fine metal and alloy powders



Patented Sept. 2, 1941 MANUFACTURE AND PRODUCTION OF FINE METAL ANDALLOY POWDERS Alan Richard Powell, London, England, assignor to Johnson,Matthey & Company, Limited, London County, England, a British company NoDrawing. Application October 2'7, 1938, Serial In Great Britain December20,

' 4 Claims. (cl. 75-05 This invention relates to the production, in theform of fine powders, of metals or alloys capable of being produced bythe conversion of their compounds into metal by a process of reductionor dissociation carried out at a temperature below the melting point ofthe metals or alloys. If powdered reducible compounds of these metalsare subjected to reduction by heating in the presence of reducingagents, or are dissociated by heating, the particles of the metalsformed coalesce to form larger aggregates which it is impossible tocomminute by mechanical means by reason of their malleability.

According to this invention the said metals or alloys thereof areobtained in the form of fine powder by carrying out the dissociation orreduction of one or more suitable compounds of these metals in thepresence of a suitable distributing or dispersing agent (hereinaftertermed a distributing agent) whlchis capable of being removed withoutdetriment to the powdered meta1 or metals or alloys formed and thenremoving the distributing agent.

Metals which can be produced in powdered form by the process of thisinvention include copper, silver, gold, iron, nickel, cobalt, and theplatinum group metals or alloys of two or more of these metals or alloysof one or more of these metals with one or more fusible metals (as forexample, tin, lead, zinc or cadmium) which can a be reduced below themelting point of formed.

For example an oxide, hydroxide or carbonate of one of the said metalsmay be co-precipitated with the oxide, hydroxide or carbonate of anothermetal incapable of reduction or dissociathe alloy tion, for example ofthe alkaline earths or magv nesium, the resulting mixed precipitatewashed free from soluble, salts, dried and then subjected to adissociation or reduction treatment whereby the said metal compound isconverted to metal which remains in a finely divided state evenlyvdistributed throughout the finely divided oxide of the other metalconstituting the distributing agent. The distributing agent is thenremoved, for example by leaching out with a suitable solvent. In somecases, the finely divided metal may be floated away from thedistributing agent by the use of a liquid of suitable specific gravityor by the use of the usual oil flotation process.

. Alternatively the distributing agents may be impregnated with asolution of a suitable combeing worked up in the same way as the mixedprecipitate referred to above.

The temperature and duration of the dissociation or reduction treatmentinfluences the grain size of the resulting metal powder, the powderstenging to be finer the lower the temperature use The proportion ofdistributing agent used also tends to influence the size of theparticles of the resulting metal powder, the particles being smaller shelarger the amount of distributing agent use According to this inventionit is possible to obtain metal powders in such a state of fineness thatthey will remain suspended in Water for relatively long periods.

The fine metal powders obtainable according to this invention may beused for example for the decoration of ceramic ware, for the productionof conducting surfaces on non-conducting materials, for the preparationof metallic paints and also in powder metallurgy.

When using a mixture of two or more compounds of the said metals, as forexample silver and copper nitrates, it is possible by selecting asuitable temperature in the dissociation or reduction treatment toobtain a powdered alloy because by reason of the very fine state ofsubdivision of the metals, difiusion takes place rapidly during theheating. It is thus possible to obtain such alloys in the form of veryfine powders.

While in the case of tin, for example, powdered I metal cannot beobtained by the method according to this invention, since at thetemperature necessary for reduction, namely at least 600 centigrade, themetal melts and runs together,

I have found that by using a tin compound in admixture with a metalcompound capable of reduction below the melting point of the alloyformed, there is formed by thereduction treatpound .of the metal, theresulting mixture then ment a powdered alloy; this is possible becausethe melting point of the alloy formed from they tin and the other metalor metals is higher than that of tin. Other metals may also be added inthe form of'reducible compounds, so that alloys are obtained in the formof fine powders. For example with copper or precious metal compounds,tin compounds may be used in any proportion provided that the alloyproduced melts at above the reduction temperature (the temperature ofthe reduction treatment being above 600 centigrade) and cadmium and zinccompounds may be used in amounts of not more than about per cent byweight (the temperature of the reduction treatment being for examplebebetween 700 and 800 centigrade). If larger amounts of cadmium or zincare used, volatiiisation occurs.

Iron, cobalt, nickel, gold and the platinum metals, or alloys of thesemetals in any proportion may also be obtained in the form of powders bythe process of this invention. In these cases the reduction treatment iseffected at a temperature above the reduction temperature of the metalcompounds used. Alloys of silver and copper, alloys of gold and nickel,copper, iron or platinum, or'various carat gold alloys containing notmore than 10 percent of zinc or cadmium may likewise be prepared in theform of flne powders.

The powdered alloys according to this invention are very useful forexample for solderin parts which are difllcultly: accessible and formany other purposes.

The following examples will further illustrate how the said inventionmay be carried out in practice, but the invention is not restricted tothese examples.

Example 1 A mixture of silver nitrate and calcium or magnesium nitrate(corresponding to a ratio by weight of AgzCaO or Mg0=1:l) is dissolvedto give a 10 per cent aqueous solution and the solution poured into anamount of a boiling solution of sodium carbonate or caustic soda morethan sumcient to precipitate the corresponding carbonates or hydroxides.The resulting precipitate is washed with hot water to remove solublesalts, and dried. The resulting mixture is then subjected to adissociation or reduction treatment as follows. For the purposes ofdissociation, the mixture is heated in air at a temperature of from 300to 900 centlgrade. temperatures may be used but in this case the silvermelts and forms minute spheres. The reduction treatment may be carriedout by heating the mixture, for example in an atmosphere of hydrogen, ata temperature of from 200 to 900 centigrade. The product of either thedissociation or.. reduction treatment is a finely divided silver powderhomogeneously dispersed throughout the distributing agent, i. e.,calcium or magnesium carbonate or oxide. The distributing agent is thenseparated, for example by leaching with a suitable dilute acid, such asacetic acid.

The resulting silver powder, especially when the lower temperaturesmentioned are used, is extremely fine and will remain in suspension inwater for an appreciable time, and shows no tendency to cake togetherwhen it is washed and dried.

Example 2 Fine copper powder is prepared in a manner similar to that.described in Example 1, but in this case the metal oxide must besubjected to a reducing treatment since copper oxide does not dissociateon heating. The starting materials may be mixtures of copper and calciumnitrates, copper and magnesium sulphates or copper and magnesiumchlorides.

Example 3 For the production of an alloy of silver and copper in thefinely powdered form a mixture of silver and copper nitrates in such aproportion as to give the desired'copper-silver ratio in theHigheralloy, is. dissolved in water to give a 10 per cent solution,which is mixed with an equal volume of a 10 per cent aqueous solution ofcalcium .or magnesium nitrate, and the mixed solution is poured into anamount of a boiling solution of sodium carbonate or caustic soda morethan suiiicient to precipitate the corresponding carbonates orhydroxides. The resulting precipitate is washedwith hot water, dried andsubjected to a reduction treatment at a temperature of from 300-900centigrade, in an atmosphere of a reducing gas, as for example hydrogen,water gas, or producer gas, whereby all the copper and silver compoundsare reduced to the corresponding metals. In this way asilver-copperalloy is obtained in the finely powdered form, intimatelymixed with calcium or magnesium oxides. The distributing agent is thenremoved, for example by leaching with a suitable dilute acid such asacetic acid or sulphuric acid.

Example 4 Fine gold and. fine platinum and other platinum metals may beobtained by impregnating an excess of calcium or magnesium carbonateswith solutions of the chlorides of the metals, drying the resultingmixture and heating itin air or in the presence of a reducing agent toproduce Example 5 To produce an alloy of gold and platinum in the finelydivided form, a mixture of the chlorides of the two metals in thedesired proportions is dissolved in a small quantity of water and thesolution mixed with a large excess of calcium or magnesium carbonates toproduce a damp powdery mixture, which is then dried and heated in air atabout 1000 centigrade for half to one hour. After cooling, the calciumor magnesium oxides and chlorides, which have acted as the distributingagent, are remo ed by leaching with dilute hydrochloric acid. There isthus obtained an extremely finely powdered gold-platinum alloy which canbe used in powder metallurgy.

Example 6 clentcarbonate to precipitate the iron, copper,

nickel and magnesium as carbonates.

The precipitate is washed and dried and then heated in an atmosphere ofhydrogen at about 800 centlgrade. The product consists of a finelypowdered alloy of iron, copper and nickel homogeneously distributed inmagnesium oxide. The latter is removed by extraction with dilutesulphuric acid leaving the alloy in a very finely divided state. Thismay be used as a magnetic powder.

A similar method may be used for production of iron, nickel and cobaltin the form of fine powders. In this case the mixture which is pouredinto the sodium carbonate solution conor cobalt in a very fine powderedform suitable for use in powder metallurgy.

Alloys of other metals may be produced in a corresponding manner.

In the present specification and the appended claims I mean byconversion operation the operation of converting a compound of a metalor a mixture of compounds of a plurality of metals into thecorresponding metal or alloy by reduction with a suitable reducing agentor by heating to such a temperature that the compound or compounds is orare dissociated to leave the free metal or alloy. This latter occursonly in the case of compounds of the precious metals, 1. e. gold, silverand the platinum group metals.

By distributing agent I mean a substance which prevents the aggregationor sintering of the particles of a metal or metals into relativelycoarse particles. Usually when a metal oxide is reduced by hydrogen orother gaseous or solid reducing agents the fine particles of metal firstformed coalesce into much coarser particles or grow together to formrelatively large crystals. The purpose of the distributing agent is toprevent this sintering or grain growth and to keep the reduced metal inthe form of a very fine powder.

By capable of being removed without detriment to the powdered metal ormetals or alloys formed I mean that the substanceusedas a distributingagent can be dissolved in solvents which do not attack the finelypowdered metal, metals or alloy or can be removed by mechanicalprocesses, such as flotation or treatment with a heavy liquid, withoutcontaminating the finely'powdered metal, metals or alloy produced by theconversion operation.

Where in the claims the expression metal compound or "compound of ametal" is used such expression is intended to include one or more metalcompounds or the compounds of one or more metals, since, as pointed outin the examples, where a single metal compound is used the product is asingle metal and where more than one metal compound is used the productis an alloy.

What I claim is:

temperature below its melting point, which consists in forming bycoprecipitation from a solution of suitable salts an intimate mixture ofa water-insoluble compound of .the metal and a distributing agent whichprevents aggregation of the particles of the metal and is capable ofbeing removed without detriment to the metal,

subjecting the coprecipitated mixture to the con-' version operation forconverting the whole of the metallic content into a fine metallic powderand then removing the distributing agent.

2. A process of producing a fine metallic powder which consists insubjecting a solution containing a salt of a metal capable of beingproduced from its compounds by a conversion operation at a temperaturebelow its melting point and a salt of a metal selected from the groupalkaline earths and magnesium, to a precipita tion operation to producean intimate mixture of an insoluble compound of the first-named metalwith a distributing agent selected from the group oxides, hydroxides,and carbonates, of metals selected from the group magnesium and thealkaline earths, subjecting said'mixture to the conversion operation ata temperature below the melting point of the first-named metal, and

1. A process of producing in the form of fine I powder a metal capableof'being produced from its compounds by a conversion operation at a thenremoving the distributing agent.

3. A process of producing in the form of a fine powder a metal capableof being produced from its compounds by a conversion operation at atemperature below its melting point which consists in forming bycoprecipitation from a solution of suitable salts an intimate mixture ofa water-insoluble compound of the metal and a distributing agent whichprevents aggregation of thepar'ticle's of the metal and is capable ofbeing removed without detriment to the metal, subjecting thecoprecipitated mixture to the conversion operation in the presence of areducing agent and then removing the distributing agent. Y

4. A process of producing a fine metallic powder which consists informing by a precipitation operation an intimate mixture 01' a compoundof a metal capable of being produced from its compounds bya conversionoperation at a temperature below its melting point with a distributingagent which prevents aggregation of the particles of metal andis-capable of being removed without detriment to the metal, subjectingthe resulting intimate mixture of insoluble metal compoundanddistributing agent to the conversion operation in the presence of areducing agent and then removing the distributing agent.

ALAN RICHARD POWELL.

